Stable nitroxides like TEMPO are commonly used polarizing agents in high-field, solid-phase DNP. With concentrations of TEMPO on the order of 40 mM, augmentation factors of polarization, ε, on the order of 40 have been observed for the 13C urea nucleus. If the concentration is decreased, the augmentation factor falls rapidly as the S1 dipole/S2 dipole interaction becomes less efficient. R. G, Griffin demonstrated (R. G. Griffin et al.: J. Am. Chem, Soc. 126, 10844-10845, 2004) that if the two spins S1 and S2 are introduced in the same molecule, the nuclear polarization obtained is more efficient, even for lower concentrations of polarizing agent. He used various dinitroxides (FIG. 2) formed by two TEMPO units connected via different types of linking arms, such as bis-TEMPO-n-ethylene glycol (BTnE) (n indicates the number of ethylene glycol units), bis-TEMPO-Urea (BTurea), bis-TEMPO-oxalamide (BTOXA) and in particular 1-(TEMPO-4-oxy)-3-(TEMPO-4-amino) propan-2-ol (TOTAPOL).
The polarization enhancement factors observed (at 90 K, for a field of 5 T and an irradiation strength of 1.5 W) for the nucleus of 13C urea (R. G. Griffin et al: J. Am. Chem. Soc, 128, 11385-11390, 2006; J. Chem. Phys., 128, 052302-1/052302-17, 2008) went from ε=175±20 with BT2E (5 mM) or ε=190±20 with TOTAPOL (5 mM), whereas ε=40±5 only with TEMPO (40 mM).
In theory, for these dinitroxides under the working conditions used, the equality of ω2/2π−ω1/2π=ωn/2π will be practically verified for molecules that adopt a conformation in which the nodal plans of the κ systems of the two aminoxyl groups create nearly a 90° angle between them. This is a result of the strong anisotropy of TEMPO'S Lande g-factor (gxx=2.0090; gyy=2.0061; gzz=2,0021). However, these molecules have no particular structural constraints allowing the molecular geometry to be blocked such that the nodal plans of the two aminoxyl groups of the two aminoxyl units create an angle near 90° between them. Thus, the flexibility of the linkage between the two nitroxide units of these various nitroxides does not favor optimal polarization.